The present invention relates to a device at a sewing machine for applying a tensile stress in a material to be fed in a sewing process. In particular, the device is provided with several pneumatic cylinders arranged in parallel and including a control with control valves for digitally controlling the pneumatic cylinders in order to generate a variable force acting upon at least one friction element.
From U.S. Pat. No. 3,877,405 a device for tensioning the thread to be fed for sewing is known. This known device is provided with air pressurizable cylinders, which are actuated by the control depending on certain sewing parameters in order to vary the generated tensile stress in the thread. The known device, however, renders possible to vary only the tensile stress by a few values. Moreover, considerable space is required, which reduces the clearance under the sewing machine arm.
From the German Offenlegungsschrift No. 2,157,016 it is known to provide elements at a thread tensioning device which make possible a reproducable fine adjustment. This tensioning device has a limited range of adjustment and does not make possible quick repetitive finding of which is required with frequent changes of materials to be stressed.